1. Field of the Invention
This invention relates generally to medicine and more specifically to an artificial heart valve.
2. Description of the Prior Art
When a person's heart is damaged by either a congenital malformation or a disease, a surgical procedure is often used to excise a defective valve from the heart and implant an artificial heart valve. The first time the procedure was used was in 1961.
The most often used artificial heart valves fall into four major categories and have been implanted with varying degrees of success. The four categories include what are known as a ball valve, a tilting disc valve, a leaflet valve and a tissue valve.
The ball valve, which was implanted during the early years of the procedure, is of a type reported to have been used as a stopper on bottles during the nineteenth century. The ball valve includes an annular metal ring, known as an orifice ring, attached to a cage that houses either a solid or a hollow ball. The orifice ring is connected on its outer surface to what is known as a suture ring, which is typically made from a fabric. The suture ring is sewn into a tissue annulus in the anatomic position of the excised defective valve.
In response to a differential blood pressure across the orifice ring in a backward direction, the ball moves to seat itself against the orifice ring to inhibit a flow of blood in the backward direction. In response to a differential blood pressure across the orifice ring in a forward direction, the ball moves away from the orifice ring to allow a flow of blood in the forward direction.
The ball valve is inherently symmetrical. Because of the symmetry, the blood is symmetrically dispersed about the ball when flowing through the orifice ring, thereby providing a spatially symmetrical velocity distribution of blood around the ball. Because of the inherent symmetry there is no preferred orientation about the axis of the orifice ring when the ball valve is implanted. Therefore, the inherent symmetry is a desirable feature of the ball valve.
The ball valve has undesirable features, such as a high profile caused by the cage being large, and the ball being moved through a large displacement to achieve a desired flow of blood. Additionally, the orifice ring must generally be thick enough to seat the ball. The thickness of the orifice ring reduces what is known as an orifice-to-annulus diameter ratio. It is desirable for an artificial heart valve to have a high orifice-to-annulus diameter ratio to decrease valvular resistance to a flow of blood. Moreover, it is documented that the movement of the ball may cause the ball valve to have a higher dislodgement rate than other types of artificial heart valves.
When, for example, the ball valve is implanted in the mitral position, there may be an undesirable ventricular impingement during a contraction of tissue around the cage. When the ball valve is implanted in the aortic position, where there is a narrow aortic root, the area between the ball and the aortic wall provides less than the desired flow of blood. In other words, in the aortic position the ball valve may have a high valvular resistance. Additionally, there may be turbulence on the outlet side of the ball valve that results in a thrombus (blood clot) being formed.
An undesirable audible noise may be emitted when the ball is positioned against the orifice ring. Moreover, there is usually stasis (insufficient blood movement) near a margin or interface where the suture and orifice rings meet. Stasis may cause a thrombus, which if dislodged, may result in an embolism in another part of the body of the person, thereby causing serious medical problems. When the person has a ball valve implant, anticoagulants are administered to increase the time required for the blood to clot, thereby reducing thromboembolic complications.
It should be understood that in the ball valve, as in all other artificial heart valves, the size depends upon the size of the tissue annulus. When the ball valve is large and the person has a low cardiac output, there is an increased probability of stasis. Conversely, when the ball valve is small and the person has a high cardiac output, there is an increased probability of turbulence. It should be appreciated that flow rate through the ball valve in the aortic position is on the order of three times the flow rate through the ball valve in the mitral position.
After the implantation, endothelial tissue ideally grows into the suture ring, up to the margin of the suture and orifice rings. The endothelial tissue is durable and serves to maintain the ball valve in the anatomic position. Completion of the endothelial growth takes from six months to two and one half years. However, the first stage of the endothelial growth is a thrombus formation.
It is well-known that endothelial tissue does not adhere to the orifice ring. The orifice ring may cause a rejection reaction whereby either an excess endothelial tissue growth or thrombus formation partially encapsulates the orifice ring, thereby interfering with the operation of the ball valve. Additionally, since the first stage of endothelial growth is the thrombus formation, the excess endothelial growth may result in a thromboembolic event.
Efforts have been made to eliminate the margin by covering the orifice ring with a fabric. When this is done, there is an incidence of fabric wear causing the fabric to be dislodged by the ball before the completion of the endothelial tissue growth, thereby causing serious medical problems.
More recently, the procedure has been used to implant what is known as a tilting disc valve, which includes a disc having a diameter substantially equal to the interior diameter of the orifice ring. The disc is tiltably connected to the orifice ring along a chord that intersects about 120 degrees of arc of the disc and the orifice ring. The orifice ring is connected to the suture ring as described hereinbefore.
In response to the differential blood pressure in the backward direction, the disc tilts about the 120 degree chord to seat itself to substantially occlude the orifice of the orifice ring to inhibit the flow of blood in the backward direction. In response to the differential blood pressure in the forward direction, the disc tilts approximately 60 degrees about the 120 degree cord. The tilting unseats the disc to form a small opening, along 120 degrees of arc of the orifice ring, and a large opening along 240 degrees of arc of the orifice ring. Blood flows in the forward direction through the small and the large openings.
Because the disc tilts only 60 degrees, the flow of blood through the large opening is deflected from a path that is parallel to the axis of the orifice ring, which may cause an undesirably high valvular resistance. Additionally, the flow of blood through the small and large openings has differing velocity distributions which may cause turbulence that results in a thrombus being formed.
Since the disc tilts about the 120 degree chord, the tilting disc valve is substantially asymmetrical. Because of the asymmetry, there is a preferred orientation about the axis of the orifice ring when the tilting disc valve is implanted. Surgeons have differing preferences of orientation. Additionally, the disc moves through an undesirably large displacement to achieve the desired flow of blood, thereby causing the tilting disc valve to have a high profile when the disc is tilted.
The tilting disc valve, like the ball valve, may emit an undesired audible sound when the disc is seated against the orifice ring. Moreover, there is often stasis across the 120 degree chord as well as near the margin of the suture and orifice rings near the 120 degree chord. When the person has a tilting disc valve implant, anticoagulants are administered to reduce thromboembolic complication of the type referred to in connection with the ball valve.
Similar to the ball valve, either the excess endothelial tissue growth or thrombus formation may partially encapsulate the orifice ring of the tilting disc valve. The partial encapsulation may seriously interfere with the operation of the tilting disc valve.
The surgical procedure has additionally been used to implant what is known as a leaflet valve. A flexible fabric-like material, in the shape of a disc is connected along a diameter to a thin, rigid rod, thereby forming a pair of semi-circular, moveable leaflets. The ends of the rod are respectively connected to diametrically opposite locations on the orifice ring.
In response to the differential blood pressure in the backward direction, the edges of the leaflets are seated against the orifice ring to inhibit the flow of blood in the backward direction. In response to the differential blood pressure in the forward direction, the leaflets tilt in opposite directions about the rod, away from the orifice ring, to allow the flow of blood in the forward direction.
Like the artificial heart valves described hereinbefore, when the leaflet valve is small, there may be turbulence on its outlet side that results in a thrombus being formed. Additionally, the leaflets do not usually move sufficiently far from the orifice ring to achieve the desired flow of blood. Moreover, there is usually stasis near the rod.
There are other types of leaflet valves where leaflets are independently hinged. A leaflet valve of this type usually has an undesired projection from the inlet side of its orifice ring.
The independently hinged leaflets may achieve a desired flow of blood in the forward direction. However, the independently hinged leaflets may not close simultaneously, thereby causing turbulence that results in a thrombus being formed. Additionally, one of the independently hinged leaflets may fail to close, thereby causing a serious medical problem. When the person has a leaflet valve implant, the anticoagulants are administered to reduce thromboembolic complications of the type referred to hereinbefore.
It should be appreciated that the leaflet valves have only two axes of symmetry. Hence, the leaflet valves are substantially asymetrical.
Often when the person has a contra-indication to anticoagulants, such as a bleeding ulcer, or pregnancy, administering the anticoagulants can be fatal. To obviate a need for the anticoagulants, the procedure is used to implant either a human, porcine or bovine semi-lunar cusp tissue valve. The suture ring is sewn to the tissue valve, thereby forming a cloth-tissue margin.
When the tissue valve is implanted, there is a reduced rejection reaction that reduces the incidence of excess endothelial growth and thromboembolic complications without administering the anticoagulants. For this reason, tissues of the tissue valve are referred to in the art as thrombogenic resistant.
Although the tissue valve obviates the need for anticoagulants, the tissue valve has disadvantages not shared by the artificial heart valves described hereinbefore. The cusps of the tissue valve may not close or open either simultaneously or completely. Additionally, the tissue valve has a high profile and may have a high valvular resistance.
The tissue valve often hardens and becomes partially inoperative, due to fibrosis and/or calcification, within three to five years of its implantation. Because of the hardening, the tissue valve becomes inoperative and is frequently replaced.
It should be appreciated that an artificial heart valve performs on the order of 40,000,000 openings and closings per year, thereby subjecting most components thereof to wear. Accordingly, tissues of the tissue valve cannot be used as a cover for the orifice ring of either the ball, tilting disc or leaflet valves to reduce the need for anticoagulants; the disc, the ball or leaflets would wear away the cover.
In accordance with the explanation given hereinbefore, implantation of an artificial heart valve of the prior art often undesirably exposes the person to risks of thrombus formation due to either stasis or turbulence, and excess endothelial tissue growth that interferes with operation of the artificial heart valve. Moreover, anticoagulants must either be administered to the person or the person must be subjected to frequent replacement of the artificial heart valve. There is a need for an artificial heart valve that has the best properties of known types of artificial heart valves without the disadvantages thereof.